1. Field of the Invention
The present invention relates to humidification of a gas, and more particularly to an improved method of humidifying a gas wherein gas and liquid are directly brought into contact with each other and the liquid is indirectly brought into contact with a third fluid so that heat contained in the third fluid is used to evaporate the liquid and increase the humidity contained in the gas.
2. Description of the Prior Art
Generally, a humidifier has been used for mixing steam into hydrocarbon gas in a process in which hydrocarbon gas and steam are mixed in a predetermined ratio and heated together with a catalyzer to be reformed in a steam reforming method to produce gas as a raw material for synthesizing ammonia or methanol.
As an example of a humidifier which has been used heretofore, there is a wetted wall type humidifier in which gas and water are directly brought into contact with each other by using a wetted wall to increase the humidity in the gas. FIG. 1 schematically shows the wetted wall type humidifier.
In FIG. 1, water 102 is supplied from a spray 105 and flows down over the upper portion of a heat conductive pipe 104 to form a liquid film on its inner surface. When the water flows down the inner portion of the pipe 104, the water is heated by heat supplied by a heating medium 103 through a wall of the pipe and evaporated.
Gas 101 is supplied through a channel inlet 106. The gas is heated and the humidity contained in the gas is increased in the pipe 104 and is then collected from a channel outlet 107.
The heating medium 103 is supplied through a side fluid inlet 108 and flows through the space surrounding the pipe to heat the fluid flowing in the pipe, and is thereby cooled. The heating medium 103 is then withdrawn from the humidifier through a side fluid outlet 109. Reference numeral 110 denotes a baffle plate, and reference numeral 111 denotes a pipe plate. While only the single pipe 104 is illustrated in FIG. 1, it is needless to say that a plurality of pipes is required to obtain sufficient effect upon implementation of the process.
FIG. 2 shows an enlarged longitudinal cross sectional view of a portion of the heat transfer pipe 104 shown in FIG. 1. Reference numerals 101-104 designate the same elements as those in FIG. 1. Reference numeral 202 denotes water film.
The conventional method and apparatus shown in FIG. 1 is disadvantageous in that the water film 202 is broken and the wall surface is dried when the amount of water is reduced too much. This phenomenon is hereinafter referred to as "the occurrence of dry patches". The occurrence of dry patches is due to the local surface tension distribution of the liquid film or the production of air bubbles by the film boiling when of being heated, and tends to be generated in the area in which the amount of water is less owing to evaporation. The occurrence of dry patches produces the following disadvantages.
(1) Concentration of the Cl.sup.- ions in water is effected upon the occurrence of dry patches and stress-corrosion cracking (SCC) may occur in the heat conductive pipe if pipe formed of austenitic stainless steel is used. (Since general carbon steel produces corrosion due to carbonic acid, stainless steel is often used.)
(2) Local thermal stress repeatedly occurs due to temperature variation by the phenomenon that the wall surface is irregularly dried and wetted, and thermal fatigue may occur in the heat conductive pipe.